具有丰富晶界的铜催化剂在气-液平衡扩散电极上高效电还原CO2制C2H4

doi:10.1016/S1872-2067(23)64540-1

摘要/Abstract

摘要：

电催化二氧化碳还原反应(CO₂RR)可以将二氧化碳转化为具有高经济价值的碳氢化合物, 被认为是实现碳中和并缓解能源危机的一种有潜力的技术. 铜(Cu)作为一种最有应用前景的非贵金属催化剂之一, 表现出较高的催化CO₂RR转化为多碳产物(C₂₊)的活性. 然而, 电催化CO₂还原成C₂₊产物涉及一个动力学过程缓慢的C-C偶联反应, 这导致C₂₊产物的选择性较低, 电流密度低, 阻碍了其在工业电解槽中的实际应用. 同时, CO₂RR产物的选择性不仅取决于热力学速率决定步骤, 还取决于传质控制动力学. CO₂RR发生在固-气-液三相反应界面, 气-液的平衡扩散可以有效抑制析氢竞争反应, 进而提高CO₂RR的反应效率.

本文设计合成了一种富晶界的Cu纳米带催化剂, 并构建了气-液平衡扩散的电极结构, 用于高效电催化二氧化碳还原制备乙烯(C₂H₄). 以一种碱式碳酸铜(Cu₂CO₃(OH)₂)纳米带为前驱体, 在原位电化学还原条件下, 前驱体中的Cu²⁺离子获得电子被还原为金属Cu, 而释放的CO₃^2-和OH^-混合阴离子调节金属Cu的生长. 生成的Cu纳米带由细小的纳米颗粒堆积而成, 并暴露出大量的由Cu(111), Cu(200)和Cu(220)晶面形成的富晶界结构(GBs). 同时, 在CO₂RR测试中发现催化剂层的厚度是影响CO₂和电解质传质的关键因素. 通过调整催化层厚度, CO₂和电解质可以同时到达催化剂表面, 参与到CO₂RR中, 实现了气-液平衡扩散, 有效抑制了氢析出副反应. 在晶界效应和气-液平衡扩散的协同作用下, 优化后的电极在电流密度为700 mA cm^-2时, 对C₂H₄和C₂₊产物的法拉第效率分别高达67.2%和82.1%. 此外, C₂H₄的部分电流密度可高达505 mA cm^-2, 高于大多数文献报道的结果. 原位拉曼光谱和衰减全内反射表面增强红外吸收光谱结果表明, 丰富的晶界结构增强了CO₂在催化剂表面的活化, 显著促进了*CO中间体的形成和吸附, 加速了C-C偶联过程形成*OCCO和*OCCOH中间体, 提高了C₂H₄和其他C₂₊产物的产率.

综上, 本文设计了一种高活性Cu催化剂和电极结构, 为高效电催化CO₂还原为C₂H₄等C₂₊产物提供参考.

关键词: 电催化二氧化碳还原, 乙烯, C₂₊产物, 晶界, 气-液扩散

Abstract:

The electrocatalytic CO₂ reduction reaction (CO₂RR) is a promising technology to produce value-added hydrocarbon chemicals, however, achieving a high selectivity to C₂₊ products at the industrial current density remains a great challenge. Herein, we demonstrate grain boundary-abundant copper (Cu) nanoribbons on balanced gas-liquid diffusion electrodes for efficient CO₂RR to ethylene (C₂H₄). The Cu(II) carbonate basic (Cu₂CO₃(OH)₂) nanoribbon is used as a precursor to convert into metal Cu under in situ electrochemical reduction. Unexpectedly, the generated Cu nanoribbon is formed by stacking tiny nanoparticles with exposure of Cu(111), Cu(200) and Cu(220) facets, which creates abundant grain boundaries (GBs). During CO₂RR test, the thickness of the catalyst layer is identified as a crucial factor for the mass transfer of CO₂ and electrolyte. By tailoring the thickness of catalytic layer, CO₂ and electrolyte can simultaneously reach the surface of catalyst and participate in CO₂RR. Under the synergetic effects of GBs and balanced gas-liquid diffusion, the optimized electrode delivers the Faradaic efficiencies toward C₂H₄ and C₂₊ products as high as 67.2% and 82.1% at the current density of 700 mA cm^-2, respectively. Moreover, the partial current density of C₂H₄ can reach up to 505 mA cm^-2, which is significantly higher than most reported results. The in situ Raman and attenuated total reflection surface-enhanced infrared absorption spectra show that abundant GBs enhance the activation of CO₂ and significantly promote the formation and adsorption of *CO intermediates, which accelerate C-C coupling to form *OCCO and *OCCOH intermediates and improve the production of C₂H₄ and other C₂₊ products.

Key words: Electrochemical CO₂ reduction, Ethylene, C₂₊ product, Grain boundary, Gas-liquid diffusion

卞磊, 张紫阳, 田昊, 田娜娜, 马智, 王中利. 具有丰富晶界的铜催化剂在气-液平衡扩散电极上高效电还原CO₂制C₂H₄[J]. 催化学报, 2023, 54: 199-211.

Lei Bian, Zi-Yang Zhang, Hao Tian, Na-Na Tian, Zhi Ma, Zhong-Li Wang. Grain boundary-abundant copper nanoribbons on balanced gas-liquid diffusion electrodes for efficient CO₂ electroreduction to C₂H₄[J]. Chinese Journal of Catalysis, 2023, 54: 199-211.

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链接本文: https://www.cjcatal.com/CN/10.1016/S1872-2067(23)64540-1

https://www.cjcatal.com/CN/Y2023/V54/I11/199

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Fig. 1. (a) Schematic illustration for the preparation of the Cu2CO3(OH)2 and ED-Cu nanoribbons catalysts. SEM images of (b) Cu2CO3(OH)2 precursor and (c) ED-Cu catalyst. (d) TEM images of ED-Cu. (e,f) HR-TEM image of ED-Cu nanoribbon. Inset in (e) and (f) is the corresponding FFT pattern.

Fig. 2. Characterization of chemical components of catalysts. (a) XRD patterns, (b) Cu 2p XPS spectra, (c) Cu LMM spectra, and (d) C 1s XPS spectra of Cu2CO3(OH)2 precursor before reduction and ED-Cu catalyst after reduction. (e) Cu K-edge XANES spectra and (f) Cu K-edge Fourier transformed EXAFS spectra of ED-Cu catalyst..

Fig. 3. (a) Cross-section SEM images of the electrodes with different catalytic layer thicknesses. (b) Current density, (c) C2H4 and (d) H2 FEs of different electrodes. (e) Schematic illustration of gas-liquid diffusion and reaction interface on electrodes with different catalyst layer thicknesses (CL) during CO2RR reaction process.

Fig. 4. CO2RR performance on ED-Cu, OD-Cu and TR-Cu catalysts. (a) LSV curves of different catalysts. Product distributions and corresponding Fes produced by (b) ED-Cu, (c) OD-Cu and (d) TR-Cu. (e) FEs of C2+ and (f) partial current density of C2H4 over ED-Cu, OD-Cu and TR-Cu at different current densities. (g) Comparisons of the FE and current density of C2H4 for CO2RR on ED-Cu with those of reported Cu-based catalysts. (h) Stability test of graphite/carbon NPs/ED-Cu/PTFE electrode at a constant current density of 700 mA cm-2 in a flow cell with 1 mol L-1 KOH.

Fig. 5. In situ Raman spectra of CO2RR in the regions of 200-1400 and 1600-2500 cm-1 on (a) ED-Cu, (b) OD-Cu, and (c) TR-Cu.

Fig. 6. In situ ATR-SEIRAS spectra of catalysts under CO2RR and reaction mechanism. (a) ED-Cu. (b) OD-Cu. (c) TR-Cu. (d) Schematic diagram of the proposed catalytic mechanism over ED-Cu catalyst.

参考文献

[1]	A. Ozden, J. Li, S. Kandambeth, X.-Y. Li, S. Liu, O. Shekhah, P. Ou, Y. Zou Finfrock, Y.-K. Wang, T. Alkayyali, F. Pelayo García de Arquer, V. S. Kale, P. M. Bhatt, A. H. Ip, M. Eddaoudi, E. H. Sargent, D. Sinton, Nat. Energy, 2023, 8, 179-190.
[2]	T. Tang, Z. Wang, J. Guan, Acta Phys.-Chim. Sin., 2023, 39, 2208033.
[3]	J. Yin, Z. Gao, F. Wei, C. Liu, J. Gong, J. Li, W. Li, L. Xiao, G. Wang, J. Lu, L. Zhuang, ACS Catal., 2022, 12, 1004-1011.
[4]	S. Nitopi, E. Bertheussen, S. B. Scott, X. Liu, A. K. Engstfeld, S. Horch, B. Seger, I. E. L. Stephens, K. Chan, C. Hahn, J. K. Nørskov, T. F. Jaramillo, I. Chorkendorff, Chem. Rev., 2019, 119, 7610-7672.
[5]	T. Tang, Z. Wang, J. Guan, Adv. Funct. Mater., 2022, 32, 2111504.
[6]	A. R. Woldu, Z. Huang, P. Zhao, L. Hu, D. Astruc, Coord. Chem. Rev., 2022, 454, 214340.
[7]	H. Yang, S. Li, Q. Xu, Chin. J. Catal., 2023, 48, 32-65.
[8]	T. Tang, Z. Wang, J. Guan, Exploration, 2023, 3, 20230011.
[9]	X. Bai, J. Guan, Chin. J. Catal., 2022, 43, 2057-2090.
[10]	C. Liu, M. Zhang, J. Li, W. Xue, T. Zheng, C. Xia, J. Zeng, Angew. Chem. Int. Ed., 2021, 61, e202113498.
[11]	M. Xie, Y. Shen, W. Ma, D. Wei, B. Zhang, Z. Wang, Y. Wang, Q. Zhang, S. Xie, C. Wang, Y. Wang,, Angew. Chem. Int. Ed., 2022, 61, e202213423.
[12]	T. N. Nguyen, Z. Chen, A. S. Zeraati, H. S. Shiran, S. M. Sadaf, M. G. Kibria, E. H. Sargent, C.-T. Dinh, J. Am. Chem. Soc., 2022, 144, 13254-13265.
[13]	Q. Wu, C. Liu, X. Su, Q. Yang, X. Wu, H. Zou, B. Long, X. Fan, Y. Liao, L. Duan, Z. Quan, S. Luo, ACS Nano, 2023, 17, 402-410.
[14]	L. Fan, C.-Y. Liu, P. Zhu, C. Xia, X. Zhang, Z.-Y. Wu, Y. Lu, T. P. Senftle, H. Wang, Joule, 2022, 6, 205-220.
[15]	Y. Wang, Z. Wang, C.-T. Dinh, J. Li, A. Ozden, M. G. Kibria, A. Seifitokaldani, C.-S. Tan, C. M. Gabardo, M. Luo, H. Zhou, F. Li, Y. Lum, C. McCallum, Y. Xu, M. Liu, A. Proppe, A. Johnston, P. Todorovic, T.-T. Zhuang, D. Sinton, S. O. Kelley, E. H. Sargent, Nat. Catal., 2020, 3, 98-106.
[16]	R. G. Mariano, K. McKelvey, H. S. White, M. W. Kanan, Science, 2017, 358, 1187-1192.
[17]	H. Jung, S. Y. Lee, C. W. Lee, M. K. Cho, D. H. Won, C. Kim, H. S. Oh, B. K. Min, Y. J. Hwang, J. Am. Chem. Soc., 2019, 141, 4624-4633.
[18]	X. Feng, K. Jiang, S. Fan, M. W. Kanan, J. Am. Chem. Soc., 2015, 137, 4606-4609.
[19]	H. Li, K. Jiang, S.-Z. Zou, W.-B. Cai, Chin. J. Catal., 2022, 43, 2772-2791.
[20]	C. W. Li, J. Ciston, M. W. Kanan, Nature, 2014, 508, 504-507.
[21]	Z.-Z. Wu, X.-L. Zhang, Z.-Z. Niu, F.-Y. Gao, P.-P. Yang, L.-P. Chi, L. Shi, W.-S. Wei, R. Liu, Z. Chen, S. Hu, X. Zheng, M.-R. Gao, J. Am. Chem. Soc., 2022, 259-269.
[22]	J. Zhang, Y. Wang, Z. Li, S. Xia, R. Cai, L. Ma, T. Zhang, J. Ackley, S. Yang, Y. Wu, J. Wu, Adv. Sci., 2022, 9, 2200454.
[23]	Z. Chen, T. Wang, B. Liu, D. Cheng, C. Hu, G. Zhang, W. Zhu, H. Wang, Z. Zhao, J. Gong, J. Am. Chem. Soc., 2020, 142, 6878-6883.
[24]	S. Kim, D. Shin, J. Park, J.-Y. Jung, H. Song, Adv. Sci., 2023, 10, 2207187.
[25]	Q. Wu, R. Du, P. Wang, G. Waterhouse, J. Li, Y. Qiu, K. Yan, Y. Zhao, W.-W. Zhao, H.-J. Tsai, M.-C. Chen, S.-F. Hung, X. Wang, G. Chen, ACS Nano, 2023, 17, 12884-12894.
[26]	C. Dinh, T. Burdyny, M. G. Kibria, A. Seifitokaldani, C. M. Gabardo, F. Pelayo Garcia de Arquer, A. Kiani, J. P. Edwards, P. D. Luna, O. S. Bushuyev, C. Zou, R. Q. Bermudez, Y. Pang, D. Sinton, E. H. Sargent, Science, 2018, 360, 783-787.
[27]	Y. C. Tan, K. B. Lee, H. Song, J. Oh, Joule, 2020, 4, 1104-1120.
[28]	J. Bi, P. Li, J. Liu, S. Jia, Y. Wang, Q. Zhu, Z. Liu, B. Han, Nat. Commun., 2023, 14, 2823.
[29]	H. Rabiee, L. Ge, X. Zhang, S. Hu, M. Li, Z. Yuan, Energy Environ. Sci., 2021, 14, 1959-2008.
[30]	X.-D. Zhang, T. Liu, C. Liu, D.-S. Zheng, J.-M. Huang, Q.-W. Liu, W.-W. Yuan, Y. Yin, L.-R. Huang, M. Xu, Y. Li, Z.-Y. Gu, J. Am. Chem. Soc., 2023, 145, 2195-2206.
[31]	J. Hu, Y. Li, Y. Zhen, M. Chen, H. Wan, Chin. J. Catal., 2021, 42, 367-375.
[32]	Y. Zhang, Y. Chen, R. Liu, X. Wang, H. Liu, Y. Zhu, Q. Qian, Y. Feng, M. Cheng, G. Zhang, InfoMat, 2023, 5, e12375.
[33]	T. Moller, F. Scholten, T. N. Thanh, I. Sinev, J. Timoshenko, X. Wang, Z. Jovanov, M. Gliech, B. Roldan Cuenya, A. S. Varela, P. Strasser, Angew. Chem. Int. Ed., 2020, 59, 17974-17983.
[34]	F. Bienen, A. Löwe, J. Hildebrand, S. Hertle, D. Schonvogel, D. Kopljar, N. Wagner, E. Klemm, K. A. Friedrich, J. Energy Chem., 2021, 62, 367-376.
[35]	W. Zhang, C. Huang, Q. Xiao, L. Yu, L. Shuai, P. An, J. Zhang, M. Qiu, Z. Ren, Y. Yu, J. Am. Chem. Soc., 2020, 142, 11417-11427.
[36]	Y. Zhou, Y. Yao, R. Zhao, X. Wang, Z. Fu, D. Wang, H. Wang, L. Zhao, W. Ni, Z. Yang, Y.-M. Yan, Angew. Chem. Int. Ed., 2022, 61, e202205832.
[37]	L. Wang, X. Li, L. Hao, S. Hong, A. Robertson, Z. Sun, Chin. J. Catal., 2022, 43, 1049-1057.
[38]	B. Kim, Y. C. Tan, Y. Ryu, K. Jang, H. G. Abbas, T. Kang, H. Choi, K.-S. Lee, S. Park, W. Kim, P. P. Choi, S. Ringe, J. Oh, ACS Energy Lett., 2023, 8, 3356-3364.
[39]	C. M. Gabardo, A. Seifitokaldani, J. P. Edwards, C.-T. Dinh, T. Burdyny, M. G. Kibria, C. P. O’Brien, E. H. Sargent, D. Sinton, Energy Environ. Sci., 2018, 11, 2531-2539.
[40]	J. Zhang, C. Guo, S. Fang, X. Zhao, L. Li, H. Jiang, Z. Liu, Z. Fan, W. Xu, J. Xiao, M. Zhong, Nat. Commun., 2023, 14, 1298.
[41]	W. Ma, S. Xie, T. Liu, Q. Fan, J. Ye, F. Sun, Z. Jiang, Q. Zhang, J. Cheng, Y. Wang, Nat. Catal., 2020, 3, 478-487.
[42]	J. Du, C. Zhou, Z. Yang, J. Cheng, Y. Shen, X. Zeng, L. Tan, L. Dong, Surf. Coatings Technol., 2019, 36, 282-290.
[43]	P. Li, J. Bi, J. Liu, Y. Wang, X. Kang, X. Sun, J. Zhang, Z. Liu, Q. Zhu, B. Han, J. Am. Chem. Soc., 2023, 145, 4675-4682.
[44]	X. Huang, D. Wang, S. Yan, P. An, J. Han, Z. Guo, X. Li, Z. Chen, L. Chang, S. Lu, Z. Tang, Nano Res., 2022, 15, 7910-7916.
[45]	Z. Wang, Y. Li, X. Zhao, S. Chen, Q. Nian, X. Luo, J. Fan, D. Ruan, B.-Q. Xiong, X. Ren, J. Am. Chem. Soc., 2023, 145, 6339-6348.
[46]	G.-Y. Duan, X.-Q. Li, G.-R. Ding, L.-J. Han, B.-H. Xu, S.-J. Zhang, Angew. Chem. Int. Ed., 2021, 134, e202110657.
[47]	Z.-Y. Zhang, H. Tian, L. Bian, S.-Z. Liu, Y. Liu, Z.-L. Wang, J. Energy Chem., 2023, 83, 90-97.
[48]	J. Sang, P. Wei, T. Liu, H. Lv, X. Ni, D. Gao, J. Zhang, H. Li, Y. Zang, F. Yang, Z. Liu, G. Wang, X. Bao, Angew. Chem. Int. Ed., 2022, 61, e202114238.
[49]	B. Zhang, J. Zhang, M. Hua, Q. Wan, Z. Su, X. Tan, L. Liu, F. Zhang, G. Chen, D. Tan, X. Cheng, B. Han, L. Zheng, G. Mo, J. Am. Chem. Soc., 2020, 142, 13606-13613.
[50]	J. Cheng, L. Chen, X. Xie, K. Feng, H. Sun, Y. Qin, W. Hua, Z. Zheng, Y. He, W. Pan, W. Yang, F. Lyu, J, Zhong, Z. Deng, Y. Jiao, Y. Peng, Angew. Chem. Int. Ed., 2023, 62, e202312113.
[51]	Y. Shi, Y. Wang, C.-L. Dong, T. T. Thuy Nga, D. Wei, J. Wang, X. Zhao, M. Wang, K. Zhang, M. Li, F. Dong, S. Shen, Adv. Energy Mater., 2023, 13, 2203896.
[52]	R. Du, Q. Wu, S. Zhang, P. Wang, Z. Li, Y. Qiu, K. Yan, G. I. N. Waterhouse, P. Wang, J. Li, Y. Zhao, W.-W. Zhao, X. Wang, G. Chen, Small, 2023, 19, 2301289.
[53]	C. Liu, X.-D. Zhang, J.-M. Huang, M.-X. Guan, M. Xu, Z.-Y. Gu, ACS Catal., 2022, 12, 15230-15240.
[54]	Y. Pang, J. Li, Z. Wang, C.-S. Tan, P.-L. Hsieh, T.-T. Zhuang, Z.-Q. Liang, C. Zou, X. Wang, P. D. Luna, J. P. Edwards, Y. Xu, F. Li, C.-T. Dinh, M. Zhong, Y. Lou, D. Wu, L.-J. Chen, E. H. Sargent, D. Sinton, Nat. Catal., 2019, 2, 251-258.

具有丰富晶界的铜催化剂在气-液平衡扩散电极上高效电还原CO₂制C₂H₄

Grain boundary-abundant copper nanoribbons on balanced gas-liquid diffusion electrodes for efficient CO₂ electroreduction to C₂H₄

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